An optical or radio-frequency (RF) source is commonly referred to as a frequency comb when its spectral representation is given by multiple, equidistantly spaced frequency tones. Various mechanisms can be used to generate a frequency comb; these are often classified with respect to their spectral bandwidth, frequency stability, spectral tone purity (i.e., signal-to-noise ratio or SNR), coherence and power. Frequency combs can be used to establish a spectral reference that can be used to relate the position of a spectral absorption or emission profile in applications such as in precision ranging, spectroscopy, sensing, or the like. The spectral reference can be established either locally, when the bandwidth of the frequency comb is smaller than a frequency octave, or globally, when the bandwidth of the frequency comb is equal to or exceeds a spectral octave. Both type of frequency combs can be used in metrology, spectroscopy, clock distribution, physical ranging, and waveform synthesis, among other applications. To be practically useful, a device or means for frequency comb generation should be power efficient, possess sufficient spectral bandwidth, be characterized by power equalized spectrum across the operational bandwidth and have high degree of coherency. As a secondary requirement, frequency comb device should also be compact and capable of stable operation in unprotected environments outside of laboratory conditions.
Commonly used techniques for frequency comb generation include, in direct or indirect form, the use of optical or RF cavities to establish the frequency reference. Frequency comb generation using mode-locked lasers (MLLs) is particularly widespread, and can be used in conjunction with nonlinear process outside of an MLL cavity. An MLL source inherently represents frequency comb by itself: pulsed temporal output, when observed in the spectral domain, corresponds to a frequency comb whose spectral width is defined by a gain bandwidth of the laser medium, with the rest of parameters dictated by the specifics of the physical locking mechanism. In the temporal domain, separation between adjacent optical pulses of an MLL output is referred to as its repetition rate; in the spectral domain, an inverse of the repetition rate defines the frequency pitch (separation between adjacent spectral peaks) of the frequency comb. An MLL is often used to seed the nonlinear process in order to enhance bandwidth or other performance parameters of the frequency comb. When coupled with various feedback mechanisms, this approach has led to the demonstration of devices used in wave-forming, ranging and spectroscopy.
The use of an MLL source for frequency comb generation necessarily introduces performance limitations. The most severe limit is imposed by the stability requirement placed on the MLL cavity. In the case when the MLL cavity is not sufficiently stabilized, its output is characterized by temporal and frequency uncertainty. In the case when a nonlinear process is used to expand the bandwidth or enhance the MLL response, these fluctuations are further amplified, thus degrading the accuracy and overall performance of the frequency comb source. While many techniques for MLL stabilization were reported and developed in the past, the fundamental limit is established by physical coupling between the frequency pitch (inverse of the repetition rate) and the cavity physical size. Higher repetition rate (higher frequency pitch) generally requires a shorter physical cavity in either the optical or RF domain. Consequently, the tolerance required to control such cavity length decreases until it reaches a physical scale that cannot be physically realized.
Thus there is a need in the art for source with repeatable output pulse characteristics independent of the pulse repetition frequency.